Progression through the mitotic cell cycle is driven by the enzymatic activity of protein kinases known as the cyclin dependent kinases (CDKs). The activity of these kinases is normally kept in check in quiescent cells by another family of proteins, the cyclic kinase inhibitors, which bind and inactivate CDKs. This project will focus on a cyclic kinase inhibitor, p27kip1, which we have previous shown acts to limit proliferation in hematopoietic cells. The proposed experiments will determine whether loss of p27, in our p27 knockout mice, will lead to abnormal cell proliferation in hematopoietic stem cells. We will use both in vitro and in vivo methods to determine whether p27 normally regulates the proliferation of stem cells in response to soluble growth factors or the extracellular matrix protein, fibronectin. The potential for p27 to cooperate with other cell cycle inhibitory proteins in the control of stem cell proliferation will also be investigated.